Updating an object property doesn't work in an enumerable-CodePudding

I defined an Order class as following:

public class Order
{
    public int Id { get; set; }
    public string OrdNumber { get; set; }
}

var dict = new Dictionary<int, string>();
  
dict.Add(1, "ORD1");
dict.Add(2, "ORD2");

var orders = dict.Select(a => new Order { Id = a.Key, OrdNumber = a.Value });

I wrote following code to update Order number:

foreach (var ord in orders)
{
    ord.OrdNumber = ord.OrdNumber   "*";
    Console.WriteLine(ord.OrdNumber);
}

The code execute as I expect, the output is:

ORD1*
ORD2*

but if I add following foreach after the foreach shown above:

foreach (var ord in orders)
{
     Console.WriteLine(ord.OrdNumber);
}

the result is:

ORD1
ORD2

The complete code is :

var dict = new Dictionary<int, string>();
dict.Add(1, "ORD1");
dict.Add(2, "ORD2");

var orders = dict.Select(a => new Order { Id = a.Key, OrdNumber = a.Value });

foreach (var ord in orders)
{
   ord.OrdNumber = ord.OrdNumber   "*";
   Console.WriteLine(ord.OrdNumber);
}

foreach (var ord in orders)
{
   Console.WriteLine(ord.OrdNumber);
}

and the output is :

ORD1*
ORD2*
ORD1
ORD2

Adding .ToList() to end of following line:

dict.Select(a => new Order { Id = a.Key, OrdNumber = a.Value })

fixes the problem. The output will now be:

ORD1*
ORD2*
ORD1*
ORD2*

Can anyone explain why this happens?

CodePudding user response：

.Select() returns a IEnumerable, which is a sequence playing, in your case, a => new Order { Id = a.Key, OrdNumber = a.Value }

Every time you'll foreach the IEnumerable, you'll play that sequence.

Now, when adding .ToList() it materialize the sequence in a List, by creating and keeping the objects. When you foreach the List, you don't play its creation sequence, but what its values actually hold, that's why in that case ord.OrdNumber = ord.OrdNumber "*"; is keps when using a List